it-tools/src/tools/regex-memo/regex-memo.content.md

Normal characters

Expression	Description
. or [^\n\r]	any character excluding a newline or carriage return
[A-Za-z]	alphabet
[a-z]	lowercase alphabet
[A-Z]	uppercase alphabet
\d or [0-9]	digit
\D or [^0-9]	non-digit
_	underscore
\w or [A-Za-z0-9_]	alphabet, digit or underscore
\W or [^A-Za-z0-9_]	inverse of \w
\S	inverse of \s

Whitespace characters

Expression	Description
	space
\t	tab
\n	newline
\r	carriage return
\s	space, tab, newline or carriage return

Character set

Expression	Description
[xyz]	either x, y or z
[^xyz]	neither x, y nor z
[1-3]	either 1, 2 or 3
[^1-3]	neither 1, 2 nor 3

• Think of a character set as an OR operation on the single characters that are enclosed between the square brackets.
• Use ^ after the opening [ to “negate” the character set.
• Within a character set, . means a literal period.

Characters that require escaping

Outside a character set

Expression	Description
\.	period
\^	caret
\$	dollar sign
|	pipe
\\	back slash
\/	forward slash
\(	opening bracket
\)	closing bracket
\[	opening square bracket
\]	closing square bracket
\{	opening curly bracket
\}	closing curly bracket

Inside a character set

Expression	Description
\\	back slash
\]	closing square bracket

• A ^ must be escaped only if it occurs immediately after the opening [ of the character set.
• A - must be escaped only if it occurs between two alphabets or two digits.

Quantifiers

Expression	Description
{2}	exactly 2
{2,}	at least 2
{2,7}	at least 2 but no more than 7
*	0 or more
+	1 or more
?	exactly 0 or 1

• The quantifier goes after the expression to be quantified.

Boundaries

Expression	Description
^	start of string
$	end of string
\b	word boundary

• How word boundary matching works:
  • At the beginning of the string if the first character is \w.
  • Between two adjacent characters within the string, if the first character is \w and the second character is \W.
  • At the end of the string if the last character is \w.

Matching

Expression	Description
foo|bar	match either foo or bar
foo(?=bar)	match foo if it’s before bar
foo(?!bar)	match foo if it’s not before bar
(?<=bar)foo	match foo if it’s after bar
(?<!bar)foo	match foo if it’s not after bar

Grouping and capturing

Expression	Description
(foo)	capturing group; match and capture foo
(?:foo)	non-capturing group; match foo but without capturing foo
(foo)bar\1	\1 is a backreference to the 1st capturing group; match foobarfoo

• Capturing groups are only relevant in the following methods:
  • string.match(regexp)
  • string.matchAll(regexp)
  • string.replace(regexp, callback)
• \N is a backreference to the Nth capturing group. Capturing groups are numbered starting from 1.

References and tools

• MDN
• RegExplained